<!DOCTYPE html>
<html lang="en" xmlns:th="http://www.thymeleaf.org">
<head>
  <meta charset="UTF-8">
  <title>JavaScript 数据类型</title>
</head>
<body>
<h1>Undefined</h1>
<p>无值变量的值和数据类型是 undefined。</p>


<p id="demo"></p>

<script>
  var car;
  document.getElementById("demo").innerHTML =
          car + "<br>" + typeof car;
</script>

</body>
</html>
<!--输出结果：
undefined
undefined

任何变量均可通过设置值为 undefined 进行清空。其类型也将是 undefined。
var car = "Porsche";
car = undefined;
输出结果：
undefined
undefined
-->